Heretofore, for automobile panel parts having a complicated shape such as a side panel or a door inner panel, which are difficult to be press formed, interstitial free (IF) cold rolled steel sheets (270E, F) having superior deep drawability and stretchability and a TS of around 270 MPa, have been widely used.
In recent years, due to increasing needs of lighter weight and higher strength of automobile bodies, a high strength cold rolled steel sheet having a TS of 340 MPa or more, and particularly, 390 MPa or more, has been progressively applied to those parts which are difficult to be press formed. In addition, as is the case described above, there has also been a trend to apply a higher strength cold rolled steel sheet to inner parts or the like, in which a high strength cold rolled steel sheet has been used, so as to further reduce automobile weight by decreasing the number of reinforcement parts or by decreasing the thickness thereof.
However, when the strength of the high strength cold rolled steel sheet used in automobile panels is further increased, and the thickness thereof is further decreased, the occurrence of surface strain is remarkably increased due to the increase in yield strength YS, the decrease in work hardening coefficient n value, and the decrease in the thickness. This surface strain is a defect such as an undulation or a wrinkle brought out on a surface of steel sheet after press forming and deteriorates dimensional precision or appearance of press formed panels. Therefore, when a high strength cold rolled steel sheet is applied to parts which are difficult to be press formed such as automobile panel parts, the steel sheet must have superior resistance to surface strain and excellent stretchability, and more particularly, the steel sheet having a YS of 270 MPa or less and a n1-10 of 0.20 or more is preferably desired. Here, the n1-10 is a work hardening coefficient calculated from the stresses at strains of 1% and 10% of a stress-strain curve obtained from a tensile test.
To decrease the yield ratio YR (=YS/TS), a method has been well known, in which a Ti or Nb added steel having the amount of C and N decreased as small as possible is hot rolled and coiled at a temperature of 680° C. or more to decrease the number of precipitates containing Ti or Nb and thereby to promote grain growth at annealing after cold rolling. In addition, in Japanese Unexamined Patent Application Publication No. 6-108155 and Japanese Patent No. 3291639, methods for promoting grain growth have been disclosed in which the amounts of C and S of Ti added steel are controlled to bring about Ti(C, S) precipitates in order to suppress the formation of fine TiC precipitates.
The above-mentioned methods are effective for a cold rolled mild steel sheet having a TS of approximately 270 MPa. However, when the grain growth is promoted, the TS is also decreased simultaneously as the YS is decreased, and therefore the methods are not always effective for a high strength cold rolled steel sheet having a TS of 340 MPa or more. That is, since the decrease in TS must be compensated for by addition of alloying elements such as Si, Mn, or P, problems may arise in that a manufacturing cost is increased, surface defects take place, a YS of 270 MPa or less is not obtained, and the like. For example, when the steel sheet is strengthened by addition of Si, Mn, and P, accompanied by the grain growth of approximately 10 μm to 20 μm in grain size, the steel sheet can only be obtained having a YS approximately 10 MPa smaller than that of a conventional high strength cold rolled steel sheet, and in addition, the resistance to the occurrence of orange peel and the anti-secondary work embrittlement of the steel sheet also deteriorates.
On the other hand, in Japanese Unexamined Patent Application Publication Nos. 2001-131681, 2002-12943, and 2002-12946, methods have been disclosed in which, without promoting grain growth, the YS is decreased and the high n value is obtained. According to the methods described above, the amount of C is controlled to approximately 0.004 to 0.02%, which is larger than that of a conventional ultra low carbon steel sheet, and grain refinement and precipitation strengthenings are positively applied in order to decrease the YS by approximately 20 MPa than that of a conventional ultra low carbon IF steel sheet.
However, when a high strength cold rolled steel sheet having a TS of approximately 390 MPa or 440 MPa is manufactured by the methods described above, the YS exceeds 270 MPa, and it becomes difficult to perfectly suppress the occurrence of the surface strain.
It could therefore be advantageous to provide a high strength cold rolled steel sheet having a TS of 340 MPa or more, in which YS≦270 MPa and n1-10≧0.20 are satisfied, and a manufacturing method thereof, the steel sheet having superior surface strain resistance and press formability.